Miniature swine provide a unique preclinical model for studies of transplantation biology, including the recent availability of a highly inbred strain, in which successful transplantation of tissues and organs can be achieved without the need for immunosuppression. Previous studies using this model have demonstrated the importance of a functional, juvenile thymus for the induction of tolerance by a short course of high-dose immunosuppression with calcineurin inhibitors. In addition, we have recently shown that transplantation of an aged, involuted thymus as a vascularized thymic lobe (VTL) graft into a matched, juvenile, thymectomized host leads to both structural and functional rejuvenation of the thymus, implying that host factors extrinsic to the thymus are capable of reversing thymic involution. The major objective of this proposal is to identify cellular and humoral host elements responsible for rejuvenation, determine their relative roles in this process, and use this information to test the hypothesis that rejuvenation of an aged thymus will allow tolerance to be induced in adult animals by the same, simple metodology that has been effective for juvenile animals. Specifically, we will 1) establish baseline assays for thymic involution and rejuvenation using the vascularized thymic lobe transplantation model and determine the importance of the host environment for inducing and maintaining the juvenile thymic state; 2) determine the importance of selected recipient cell populations in thymic rejuvenation; 3) determine the importance of selected hormones and cytokines of the recipient in thymic rejuvenation; and 4) examine the ability of the most promising cellular and/or molecular components identified in these experiments to reverse structural thymic aging and thereby permit tolerance to be induced in adult animals. In addition to the theoretical implications of a better understanding of the role of factors extrinsic to the thymus in determining thymic involution and rejuvenation, these studies could have practical implications for the induction of transplantation tolerance in adults.